### Story summary
- **Serial Nightmare**: Detective Captain Ke Chengyu was urgently summoned to the scene of the latest murder on his birthday and found that the method of this case was very similar to the "Butterfly Killer" series of cases three years ago, and the murderer of that case had already been killed.
- **Butterfly Reappears**: The new victim has the same butterfly markings and semen evidence as in the old case, but DNA testing shows that it does not match the dead murderer, suggesting that there may be a copycat or someone else as the real murderer.
- **Expert Assistance**: Criminal psychologist Wen Yulu was introduced to the task force. She keenly discovered the sense of ritual and psychological projection in the murderer's modus operandi, and speculated that the murderer might be "creating" some kind of artwork through crime.
- **Hidden Connection**: Ke Chengyu discovered that all the victims were related to an underground club called "Butterfly", and there seemed to be some connection behind this club with the murderer who was convicted three years ago.
- **Personal Shadow**: Ke Chengyu was forced to face the possible mistakes he made when he killed the murderer three years ago. At the same time, Wen Yulu also showed his unusual persistence in the case, and the secrets hidden by the two gradually surfaced.
